1. Field of the Invention
The present invention relates to a yaw rate detecting system for a vehicle, and more particularly to an apparatus and method for detecting the yaw rate of the vehicle for use in a vehicle motion control system.
2. Description of the Related Arts
Recently, a system for controlling a vehicle motion characteristic, particularly a turning characteristic of a vehicle, has been noted, and a vehicle motion control system which is adapted to directly control a rotating moment by controlling a difference of braking force applied to left and right wheels, is now on the market. For example, when it is determined that the excessive oversteer occurs during cornering, the braking force will be applied to a front wheel located on the outside of the curve in the vehicle's path for example, to produce a moment for forcing the vehicle to turn in the direction toward the outside of the curve, i.e., an outwardly oriented moment, in accordance with an oversteer restraining control which may be called as a vehicle stability control. When it is determined that the excessive understeer occurs while a vehicle is undergoing a cornering maneuver, for example, the braking force will be applied to force the vehicle to turn in the direction toward the inside of the curve, i.e., an inwardly oriented moment, in accordance with an understeer restraining control, which may be called as a course trace performance control. The above described oversteer restraining control and understeer restraining control as a whole may be called as a steering control by braking. Accordingly, irrespective of brake pedal operation, the braking force applied to each wheel is controlled in response to a comparison of a desired yaw rate and an actual yaw rate, for example.
In the vehicle motion control system as described above, a sensor for detecting a vehicle yaw rate is employed, as described in Japanese Patent Laid-open Publication No.5-314397, for example. It is stated in that Publication that a prior apparatus for processing a signal output from a sensor neglects compensation for drift component included in the signal output from the sensor, so that an apparatus for processing the sensor signal has been proposed so as to calculate an accurate yaw rate irrespective of the drifted amount of the yaw rate sensor. In practice, a zero point signal of the yaw rate sensor is renewed to provide such a relationship that each value of steering angle of positive or negative value within a certain time period and each value of yaw rate of positive or negative value within a certain time period will coincide with each other, under a certain running condition. Also proposed is such a method that an output of the yaw rate sensor is renewed to be zero, in the case where an estimated generating yaw rate estimated in response to the output of the yaw rate sensor, e.g., steering angle, speed in the longitudinal direction, and vehicle characteristic, is smaller than a predetermined allowable yaw rate error, under such a running condition that the yaw rate is not generated, as in the case where the vehicle is running on a straight road at a low speed.
According to the apparatus for processing the sensor signal, however, an output of a steering angle sensor with low resolution is directly used for renewing the zero point. Therefore, it is impossible to set or renew the zero point signal accurately for the yaw rate sensor. Particularly, in calculating a vehicle side slip angle for use in the steering control by braking, errors in the output of the steering angle are accumulated to cause a large error. Furthermore, the process for correcting the error of the zero point by means of the apparatus for processing the output of the sensor as disclosed in the Publication is complicated, so that a relatively long processing time is needed.